STRUCTURE OF THE STEM. 



61 



83. Stem of Conifers.^ — Sketch the end of a cut-ofi billet of hard 

 pine or red cedar. Study the cut surface with a magnifying glass and 

 decide whether any of the parts readily found in the wood of the coarser- 

 grained hard-woods are absent from coniferous wood. 



Under a power of 100 or more diameters it is easy to see what it is 

 that marks off one annual ring from another. 



Study the section, compare it with 

 Fig. 50, and state the difference between 

 spring wood and fall wood. 



Sketch the whole cross -section, 

 moderately magnified 



Examine longitudinal sections, both 

 radial and tangential, of pine, spruce, fir, 

 or red cedar.^ 



Sketch a radial section and a tangen- 

 tial one, labeling the medullary rays and 

 the cells of the wood, w;ith their circular 

 markings, as shown in Fig. 51. 



84. The Early History of the 

 Stem. — In the earliest stages of 

 tlie growth of the stem it consists 

 entirely of thin-walled and rapidly 

 dividing cells. Soon, however, the 

 various kinds of tissue which are 

 found in the full-grown stem begin 

 to appear. 



In rig. 52 the process is shown 

 as it occurs in the castor bean. 

 At m, in B, is the central column 

 of pith, surrounded by eight fibro- 

 vascular bundles, fv, each of which 

 contains a number of ducts arranged 



in a pretty regular manner and surrounded by the forerunners 

 of the true wood-cells. 



1 That is, of the cone-hearing trees (mostly eyergreens), snch as the pines, spruces, 

 cedars, larches, and so on. 



^ Pine shows the large circular pits very plainly, while red cedar shows the mednl- 

 lary rays most clearly, since nearly all its red color lies in these. 



Fig. 52. — Transverse Section 

 through the Caulicle of the Castor- 

 Oil Plant at Various Stages. 

 -4, after the root has just appeared 

 outside the testa of the seed ; -B, 

 after the caulicle is nearly an inch 

 long ; C, at the end of germina- 

 tion ; r, cortex (undeveloped 

 hark) ; m, pith ; si, medullary 

 rays ; /i', fibro-vascular bundles ; 

 c&, layer of tissue which is to 

 develop into cambium. (Consider- 

 ably magnified.) 



